1. Field of the Invention
The present invention relates to an optical element and a lithographic apparatus.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. including part of one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
In a lithographic apparatus the size of features that can be imaged onto the substrate is limited by the wavelength of the projection radiation. To produce integrated circuits with a higher density of devices, and hence higher operating speeds, it is desirable to be able to image smaller features. While most current lithographic projection apparatus employ ultraviolet light generated by mercury lamps or excimer lasers, it has been proposed to use shorter wavelength radiation, e.g. of around 13 nm. Such radiation is termed extreme ultraviolet (EUV) or soft x-ray, and possible sources include, for example, laser-produced plasma sources, discharge plasma sources, or synchrotron radiation from electron storage rings.
In EUV lithography systems, submicron optical structures are present, facing the non-perfect vacuum. They are, for example, present on the surface of optical elements, e.g. EUV optical elements, like spectral purity filters (e.g. gratings), on reticles, and on image sensors. All these components are illuminated with EUV radiation. However, these structures get contaminated, for example, by debris particles from the radiation source or other particles that may be present in the non-perfect vacuum. For example, in a lithographic projection apparatus for EUV applications, it is necessary to prevent any stray particles or debris, that may be present in the apparatus, from reaching and becoming stuck to the reticle as they will otherwise be imaged on the wafer and can be printed in the final device. A too high level of contamination of the mask can lead to defect devices. Reticles can generally not be cleaned, or if cleanable, can only be cleaned a limited number of times.
In lithographic projection apparatus using relatively long wavelength ultraviolet radiation, particles are prevented from reaching the mask by a pellicle. A pellicle is a thin membrane transparent to the radiation used in the projection beam of the lithographic apparatus and located parallel to, but spaced from, the mask. Contaminant particles moving towards the mask contact and stick to the pellicle. To ensure that the particles stuck to the pellicle are not printed on the substrate, the pellicle is spaced from the mask by a distance greater than the depth of focus at mask level. However, state of the art and commercially available filters and pellicles are typically characterized by a relative high absorption for EUV radiation, when used in EUV applications, e.g. at 13.5 nm wavelength. Usually they also have a mechanical supporting structure consisting of a mesh of thin wires, whereby the wires also absorb EUV radiation. Hence, state of the art pellicles are not useful in EUV applications for protecting the mask.
Gratings can get contaminated too. Gratings for EUV radiation may include laminar sawtooth or square wave profiles, wherein on a mirror protrusions having a sawtooth or square wave profile, respectively, are laminarly arranged. See, for example, U.S. Pat. No. 6,678,037, the entire contents of which are herein incorporated by reference. Between such protrusions, contaminant particles may be collected, which are not easily removed.
In addition to gratings, mirrors, especially tilted multilayer stack mirrors (see for example Seely et al., Applied Optics 40, vol. 31, page 5565 (2001)), also have a profile with cavities and elevations, leading to the same problems of contamination and cleaning as mentioned above.
Further, lithographic projection apparatus may include an image sensing device, which may be mounted on the substrate table. Such an image sensing device is used to measure a mark pattern present in the patterning device, so as to determine a plane of best focus of the projection system (“lens”), lens aberrations and to align a substrate table with respect to the patterning device. Presently, an image sensor includes several separate sensors located behind detection structures that may take the form of gratings. See, for example, U.S. Pat. No. 6,747,282, the entire contents of which are herein incorporated by reference. Between these structures or protrusions contaminations may also be collected, which are not easily removed.